A cylinder press printer, as a high-speed screen printer (hereunder referred to as “cylinder press”) is capable of high-speed printing and has high productivity, and such printers are widely used for high-speed mass production of various types of posters made by color separation printing, which is characterized by satisfactory durability and thick film printing capability, both advantages of screen printing, high-speed mass production of decorating panels and packages of game devices, household electrical appliances and the like, and high-speed mass production of dummy prints incorporated into automatic vending machines and the like.
On the other hand, at printing manufacturers that do not maintain cylinder presses due to the high cost of the equipment, it is a common strategy to use a semi-automatic screen printer for high-speed mass production, wherein screen printing is normally achieved with a squeegee speed (corresponding to the printing speed) of no greater than about 300 mm/sec, set at a squeegee speed of about not less than 350 mm/sec as the printing speed for high-speed printing.
Now, a cylinder press will be described as a typical example of a high-speed screen printer. Since a cylinder press is capable of high-speed mass production, attempts have been made recently to use cylinder presses for creation of screen printed matter, even for graphical decorative prints such as images on vehicle meters that have increased variation and higher definition of design, household electrical appliance labels having fine character images and the like, and frames or packages of tablet devices.
A cylinder press performs printing at usually 800 to 1500 rotations per hour (number of times printed per 1 hour), so that the squeegee and scraper movement is rapid (about 350 mm per second or faster), and therefore the screen ink used for the cylinder press is an ink for screen printing having a low-viscosity property allowing the ink to follow the high-speed movement of the squeegee and scraper on the screen printing plate and to be evenly and sufficiently present on the plate.
However, when a screen ink simply having such a low-viscosity property is used for printing, it tends to result in spreading and bleeding in printed images due to the low viscosity of the ink, and in printed matter having both a wide-area solid pattern (for example, an approximately 10 cm-square quadrilateral shape) and a fine pattern (for example, an approximately 100 μm-diameter dot pattern or a 100 μm-line width fine line pattern), even if the visibility of the solid pattern is smooth and satisfactory, the spreading or bleeding occurring at the image edge sections makes it difficult to achieve sharpness, and with fine patterns, bleeding or thickening of the image can result, thereby impairing the visibility. Such problems become particularly pronounced especially with increased number of prints such as 10, 20, 30, 50 or 100 sheets.
One method used to solve the problem is to increase the viscosity of the ink, but when a high-viscosity ink that simply has increased viscosity is screen printed with a cylinder press, the rapid squeegee and scraper speed can prevent the ink from being able to be present in a sufficient and evenly spread state over the screen printing plate, resulting in defects such as chipping or poor smoothness in printed images. On the other hand, when screen printing is carried out using such high-viscosity screen ink, not with a cylinder press but with a flat bed screen printer (semi-automatic screen printer), wherein the ordinary printing speed is about 600 rotations per hour (a squeegee speed of about 300 mm/sec), the sharpness of printed images is improved but the printing speed is slow and problems are encountered in terms of short-period productivity.
Under such circumstances, when printed matter having a design comprising both a solid pattern and a fine pattern is created by high-speed screen printing, it is common to carry out the printing in two steps despite the disadvantage of having an increased number of printing steps, the steps being first printing a solid pattern alone by a step of printing the solid pattern with ink adjusted to reduced viscosity of a suitable level allowing printing of attractive solid patterns, followed by printing of a fine pattern by a step of a fine pattern alone with ink adjusted to an increased viscosity of a suitable level such that significant spreading, thickening or bleeding does not occur in the fine pattern, while it has been difficult to create printed matter of consistently stable quality since inks with adjusted viscosity suited for each step are used.
However, even with high-speed screen printing of printed matter having designs that are combinations of solid patterns and fine patterns, there is a strong demand in the market for ink having a flow property that allows printing of printed matter of consistently stable quality in a single step, and the importance of the present invention is that it leads to a solution to this problem.
Prior Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2010-047716) discloses a conductive ink composition and conductive coating film for screen printing that can form high-definition patterns, but it is an ink for printed matter comprising a combination of fine patterns and solid patterns, and moreover it includes no technical disclosure regarding the flow property useful for high-speed screen printing such as with a cylinder press.
Also, Prior Patent Documents 2 (Japanese Unexamined Patent Application Publication No. 2003-238876), Prior Patent Document 3 (Japanese Unexamined Patent Application Publication No. 2003-294930) and Prior Patent Document 4 (Japanese Unexamined Patent Application Publication No. 2012-017411) disclose flow property technology relating to ink compositions for screen printing that allow formation of high-definition patterns, but although the disclosures are of techniques for highly precise screen printing of fine patterns, as with Prior art document PTL 1, there is no technical disclosure or suggestion regarding an ink composition for satisfactory simultaneous printing of fine patterns and solid patterns in a single step using a high-speed screen printer such as a high-speed cylinder press.
Furthermore, Prior Patent Document 5 (Japanese Unexamined Patent Application Publication No. 2010-047649) discloses a composition for conductive screen printing and a technique relating to differences in the amount of change in suitable spread of the composition with respect to time based on a spread meter, but there is no technical disclosure or suggestion regarding any technique relating to high-speed screen printing, or to an ink composition for satisfactory simultaneous printing of fine patterns and solid patterns in a single step by high-speed screen printing.
Furthermore, Prior Patent Document 6 (Japanese Unexamined Patent Application Publication No. 2009-030065) discloses an ink composition for screen printing containing a resin, solvent, etc., and having excellent metallic luster or mirror surface gloss surface, and excellent adhesiveness onto base materials, but there is no technical disclosure or suggestion regarding any technique relating to high-speed screen printing, or to an ink composition for satisfactory simultaneous printing of fine patterns and solid patterns in a single step by high-speed screen printing.